The present invention relates to variable displacement hydrostatic pumps and controls therefor, and more particularly, to such pumps which are operated in response to a remote electrical input signal.
Although the present invention may be utilized with various types of pumps, it is especially advantageous when used with an axial piston pump, wherein the displacement of the pump is controlled by movement of a tiltable swashplate, and the invention will be described in connection therewith.
By way of example only, variable displacement hydrostatic pumps of the type to which the present invention relates are widely used in mobile hydraulics, i.e., on various types of moveable (mobile) vehicles. On a large percentage of the mobile vehicle applications, the variable displacement axial piston pump is controlled by a "manual controller" of the type illustrated and described in U.S. Pat. No. 4,050,247, assigned to the assignee of the present invention and incorporated herein by reference. Such a manual controller controls the communication of control pressure from a charge pump to either of a pair of stroking cylinders, which control the tilt of the swashplate and thus, the displacement of the pump, in response to manual movement of a manual input lever. Typically, the manual controller is mounted on an upper surface of the pump housing.
In certain vehicle applications, it is desirable for the vehicle operator to control the displacement of the pump at a time when the operator is nowhere near the pump. In other words, there are times when the operator needs a "remote control" for the pump. One example is on a concrete transit mixer, wherein the drum containing the concrete is rotated by means of a hydrostatic transmission located toward the forward end of the truck, and at the job site, it is frequently desirable for the transit mixer operator to be able to control drum speed while standing near the rear of the transit mixer, observing concrete flowing out of the drum.
On typical transit mixers with hydrostatic drum drives, the remote control from the operator to the pump manual controller is by means of a set of mechanical cables. Conceptually, this form of remote control is acceptable, although the typical cable arrangement is somewhat awkward and inherently limits the freedom of movement of the transit mixer operator. In addition, the mechanical cables require periodic maintenance and replacement because of normal wear and the relatively harsh environment in which the cables are used.
U.S. Pat. No. 4,183,419 discloses a hydrostatic transmission and control system, in which there is a remote electric input signal to a pump equipped with a standard manual controller. This is accomplished by locating a linear electrohydraulic actuation on top of the manual controller, with the output of the actuator connected to the manual input lever of the manual controller. Unfortunately, the arrangement in the above-cited patent results in certain parts of both of electrohydraulic actuator and the manual controller being exposed to the elements and dirt and various other foreign elements which can interfere with the reliable, long-range operation of the control.
In addition, the manner in which the electrohydraulic actuator is associated with the input to the standard manual controller may effectively eliminate the ability to provide a purely manual input to the manual controller.